[sdiy] Octave Cat modulation weirdness

[Travis Thatcher]

Thanks Nicholas for the link. I could definitely see that being
helpful in the future. Seems like there are a lot of changes from the
factory schematics on both boards.

Anyway, looking at the wiper of the OSC2 modulation pot I get the
expected attenuation of the LFO/S&H waveform depending on the pot
setting. Interestingly, I lifted the wiper of that pot, put it back
together and there was still a bit of LFO affecting OSC2. If I switch
the mod source to S&H then the modulation is very nearly not
noticeable so the synth is still usable for my purposes but I'd like
to get to the bottom of this. Using my scope I started looking for the
control signal at various points such as pin 6 of IC 24B in the OSC2
part of the schematic. I couldn't find anything, which is weird. If I
connected the mod signal at pin 3 of the mod pot to ground with a 1k
resistor, the modulation goes away, but then of course the total range
of modulation for that OSC is greatly reduced.

Like I said, the synth is still usable in certain settings but it'd
probably be good to get to the bottom of this. Also note, when I did
the previous repairs a few weeks ago the only things changes on the
right half of board A were socketed chips. No caps or resistors
replaced.

On Wed, Sep 30, 2015 at 6:28 PM, Nicholas Keller <niroke at tampabay.rr.com> wrote:
> I've seen inside four SRMs and they were all different with "fixes", some on small, stilted boards, but most of these differences were on the VCO board.  The following link won't help with your dilemma, but might help you another day... I fixed the schematics and layout images for the VCF/VCA board to reflect the actual circuits, but never got around to completing the work for the other board
>
> http://niroke.blogspot.com/2013/04/octave-cat-srm-fixed-schematics-and.html
>
>
>
>> On Sep 30, 2015, at 4:39 PM, Travis Thatcher <recompas at gmail.com> wrote:
>>
>> I totally agree that just blindly replacing stuff is not the best way
>> to go. The main work I did was on the PSU. I think I might have
>> replaced maybe 5 other tantalums on the main board since at that age
>> they seem to short pretty often... kept everything else as is. I'll
>> trace things with a scope tonight after going over everything. This
>> synth has seen a lot of work at some point in its life, either factory
>> or by other techs. There's a fair amount of jumper wires on it.
>> There's no pcb cracks or anything and they look as if they could be
>> factory mods/corrections. I've never opened up another to have a
>> comparison to.
>>
>>> On Wed, Sep 30, 2015 at 4:25 PM, Gordonjcp <gordonjcp at gjcp.net> wrote:
>>>> On Wed, Sep 30, 2015 at 01:58:27PM -0400, Travis Thatcher wrote:
>>>> I recently restored an Octave Cat SRM and it was working pretty well.
>>>> I replaced nearly all the CMOS and recapped somethings. The only thing
>>>> I couldn't perfect was the scaling when you switched from mono to poly
>>>> mode but that didn't bother me - just meant I had to adjust the tuning
>>>> of osc1 by a step.
>>>>
>>>> The weather here in central VA as well the rest of the Eastern US has
>>>> been a bit humid to say the least. Yesterday I fired it up only to
>>>> find that osc 2 is constantly getting an amount of modulation from the
>>>> LFO, even with the modulation pot turned completely off. I checked and
>>>> leg 1 of the pot still has continuity with ground. I'm guessing a cap
>>>> some where finally bit the dust, maybe egged on a bit by the humidity
>>>> in the house. Just curious if anyone has seen anything like this. I'm
>>>> going to open it up tonight to investigate.
>>>
>>> Outside of cheap crappy switched-mode power supplies, electrolytic capacitors mostly don't fail.  Disc ceramics do, and tantalum bead capacitors fail but you'd know about it from the bang and terrible smell.
>>>
>>> In nearly 30 years, I've replaced maybe about five or six electrolytics that weren't in a switched-mode PSU.
>>>
>>> What I have done is recover the aftermath of people "re-capping" things, to the point that when someone brings me a piece of gear that they say has been "re-capped" I just tell them to take it somewhere else, like the municipal tip.
>>>
>>> Go over it carefully and check your soldering, really carefully.  Clean off every bit of flux.
>>>
>>> Once you've done that, dig out the circuit diagram and try and work out where the signals are going.  Don't just blindly rip out and replace components and hope that this will fix it - more damage is done by the "HERP DERP NO WORKY MUST REPLACE CAPACITATOROSZZZ" brigade.
>>>
>>> --
>>> Gordonjcp MM0YEQ
>>>
>>>
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>>
>>
>>
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